Method for separating an azepine derivative out of a mixture containing an amine an azepine derivative

ABSTRACT

The invention relates to a method for separating by distillation a portion or the entirety of an azeptine derivative (III), which is selected from the group consisting of aminohexylidene imine, tetrahydroazepine, hexylhexahydroazepine and of aminohexylhexahydroazepine, out of a mixture (II) containing an azepine derivative (III) and an amine (I). The inventive method is characterized in that the distillation is carried out with a maximum bottom temperature of 150° C.

DESCRIPTION

The present invention relates to a process for distillative removal ofpart or all of an azepine derivative (III) selected from the groupconsisting of tetrahydroazepine, 2-aminoazepan,N-(2-azepano)-1,6-diaminohexane and N-(2-azepano)-6-aminocapronitrilefrom a mixture (II) comprising an azepine derivative (III) and an amine(I) selected from the group consisting of 6-aminocapronitrile andhexamethylenediamine, which comprises conducting the distillation at apot temperature of not more than 120° C.

Mixtures comprising an amine and an azepine derivative are customarilyobtained in the hydrogenation of nitriles to amines.

The complete hydrogenation of adiponitrile (ADN) to hexamethylenediamine(HMD), and also the partial hydrogenation with coproduction of HMD and6-aminocapronitrile (ACN), in the presence of a catalyst based on ametal such as nickel, cobalt, iron, rhodium or ruthenium is commonlyknown, for example from: K. Weissermel, H.-J. Arpe, IndustrielleOrganische Chemie, 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim,1988, page 266, U.S. Pat. Nos. 4,601,859, 2,762,835, 2,208,598, DE-A 848654, DE-A 954 416, DE-A 42 35 466, U.S. Pat. No. 3,696,153, DE-A19500222, WO-A-92/21650 and DE-A-19548289.

By-products formed include azepine derivatives such asN-(2-azepano)-1,6-diaminohexane and N-(2-azepano)-6-aminocapronitrile,especially 2-aminoazepan and tetrahydroazepine.

These azepine derivatives, which, because of their color and deleteriouseffect on product properties, constitute undesirable impurities in theamines, which are customarily used for manufacturing fibers, aredifficult to separate from the amines.

For instance, GB-A-893 709 discloses installing a delay time vessel inthe reflux line of a distillation column used for purifying HMD.

GB-A-1 238 351 describes the removal of HMD from mixtures comprising HMDand azepine derivatives, by addition of alkali metal hydroxide mixtures.

WO-A-99/48872 discloses distillatively removing azepine derivatives fromamines at overhead temperatures of from 160 to 250° C. The disadvantagewith this process is unsatisfactory separation.

Disadvantages with the processes mentioned are the use of large vessels,which makes for reduced control of the distillation columns, and theformation of solids, which can lead to blockages, and unsatisfactoryremoval of the azepine derivatives.

It is an object of the present invention to provide a process forremoving an azepine derivative from mixtures comprising an amine and anazepine derivative in a technically simple and economical manner.

We have found that this object is achieved by the process defined at thebeginning.

Suitable amines I include aromatic amines such as benzylamine, aliphaticamines such as cyclic amines, for example isophoronediamine, orpreferably acyclic amines, for example 1,4-diaminobutane, especially HMDor ACN, and also mixtures thereof.

Such amines can be prepared in a conventional manner.

For instance, HMD can be obtained by partial or complete catalytichydrogenation with a gas comprising molecular hydrogen, of ADN to HMD ormixtures comprising HMD and ACN.

Catalysts used for this hydrogenation can advantageously be those basedon a metal selected from the group consisting of ruthenium, rhodium,nickel, cobalt, preferably iron, in which case the catalysts may includefurther elements as promoters. In the case of iron-based catalysts,suitable promoters include especially one or more, such as two, three,four or five, elements selected from the group consisting of aluminum,silicon, zirconium, titanium and vanadium.

Such catalysts and the process conditions for the reaction mentioned aredescribed for example in WO-A-96/20166, DE-A-19636768 and DE-A-19646436.

Contemplated azepine derivatives III include especially 2-aminoazepan ofthe formula

N-(2-azepano)-1,6-diaminohexane of the formula

N-(2-azepano)-6-aminocapronitrile

and THA of the formula

and mixtures thereof.

The azepine derivatives (III) can be present in the mixture (II) asindividual compounds or as adducts, for example with an amine (I), inwhich case these adducts shall for the purposes of the present inventionlikewise be termed azepine derivatives (III).

Such azepine derivatives and processes for their preparation arecommonly known.

For instance, 2-aminoazepan, N-(2-azepano)-1,6-diaminohexane andN-(2-azepano)-6-aminocapronitrile and tetrahydroazepine can generally beobtained in mixtures (II) in amounts from 1 to 10,000 ppm, based on themixture, in the partial catalytic hydrogenation of ADN with a gascomprising molecular hydrogen to form HMD or mixtures comprising HMD andACN according to the process described for preparing the amines (I).Similarly, the azepine derivatives mentioned can be formed by oxidationof amines, such as HMD and ACN, for example with gases containingmolecular oxygen.

According to the present invention, the distillation is conducted withpot temperatures of not more than 120° C., preferably not more than 110°C. The distillation is advantageously carried out at pot temperatures ofnot less than 50° C., preferably not less than 80° C.

The distillation can be carried out continuously.

The distillation can be carried out batchwise.

When HMD is used as amine (I) and one or more compounds selected fromthe group consisting of AHI, HHA, AHHA and THA as azepine derivative(III), then the distillation pressure, as measured at the bottom of thedistillation apparatus, should be within the range from 1 to 300 mbar,preferably within the range from 5 to 100 mbar, especially within therange from 10 to 60 mbar.

When ACN is used as amine (I) and one or more compounds selected fromthe group consisting of 2-aminoazepan, N-(2-azepano)-1,6-diaminohexaneand N-(2-azepano)-6-aminocapronitrile and tetrahydroazepine as azepinederivative (III), then the distillation pressure, as measured at thebottom of the distillation apparatus, should be within the range from 1to 200 mbar, preferably within the range from 5 to 100 mbar, especiallywithin the range from 10 to 40 mbar.

Advantageously, amine (I) is obtained above the feed of mixture (II) tothe distillation apparatus, especially at the top of the distillationapparatus.

Advantageously the distillation provides a bottom product (VI) having ahigher weight fraction of azepine derivative (III) than mixture (II).

Suitable apparatus for the distillation is any customary distillationapparatus as described for example in Kirk-Othmer, Encyclopedia ofChemical Technology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979,pages 870–881, such as sieve plate columns, bubble cap columns orcolumns packed with arranged or dumped packing.

The distillation can be carried out in a plurality of columns, such as 2or 3, but is advantageously carried out in a single column.

In a preferred embodiment, the distillation can be carried out in twostages.

The first stage can consist of a plurality of columns, such as 2 or 3,advantageously a single column. The second stage can consist of aplurality of columns, such as 2 or 3, advantageously a single column.

Advantageously the pressure in the first stage, measured in the pot, isat least 1.5 times, especially at least double, the pressure in thesecond stage, measured in the pot.

Advantageously not less than 20% by weight of the amount fed into thefirst stage per unit time is removed from the pot of the first stage andfed to the second stage.

Advantageously the overhead product of the second stage can be recycledinto the first stage.

Advantageously the distillation mixture has added to it a compound (IV)whose boiling point is above that of said amine (I) under thedistillation conditions. Compounds (V) useful for this purpose are inparticular compounds (V) that are inert to the amine (I) under thedistillation conditions.

Useful compounds (IV) include compounds from the group consisting ofaromatics, aliphatics, such as cyclic and acyclic aliphatics, andaliphatic-aromatic compounds. These compounds may bear substituents,such as a hydroxyl, keto, ester, alkyl, aryl, cycloalkyl, arylalkylgroup, preferably a nitrile or amino group, or a plurality of identicalor different such groups.

Said compound (IV) can be a single compound or a mixture of suchcompounds.

Advantageous compounds (IV) are convertible in a simple manner, as bycatalytic hydrogenation with a gas containing molecular hydrogen, forexample, into a mixture (V) comprising an amine (I) and an amine (III)or in particular a mixture (II).

The products obtained in this conversion can be advantageously reused inthe process of the invention.

The difference in the boiling points between the amine (I) and thecompound (IV) should be from 1 to 200° C., preferably from 5 to 100° C.,under the distillation conditions.

The compound (IV) can be added to the mixture (II) before or during thedistillation.

The addition of the compound (IV) to the mixture (II) before thedistillation can be carried out in the conventional manner in customarymixing apparatuses. With this procedure, the addition of a mixture ofmixture (II) and compound (IV) into the distillation apparatus iscontemplated.

The addition of the compound (IV) to the mixture (II) during thedistillation can be effected by feeding the compound (IV) into thedistillation apparatus preferably in the bottom region.

The distillation can advantageously be carried out in the presence ofassistants which support the distillative separation of the invention,especially in the presence of carbon dioxide.

The concentration of azepine derivative (III) in the pot, based on themixture present in the pot, is not more than 0.5% by weight, preferablynot more than 0.2% by weight, especially not more than 0.15% by weight,during the distillation.

The process of the invention customarily affords the predominantproportion of azepine derivative (III) as bottom product (VI). Thisbottom product (VI) customarily includes azepine (III) in a higherweight concentration than the mixture (II) used for distillationaccording to the process of the invention (II).

Bottom product (VI) can advantageously be subjected in a conventionalmanner, for example according to the processes already mentioned forpreparing HMD or mixtures comprising HMD and ACN, to a catalytichydrogenation to obtain an amine (I), such as HMD or mixtures comprisingHMD and ACN. In the hydrogenation, azepine derivative (III) can beconverted into organic compounds, such as hexamethylimine, which mixedwith amine (I) permit removal of amine (I) in a technically simple andeconomical manner.

HMD and ACN are intermediates for industrially important polyamides,such as nylon-6 or nylon-6,6.

EXAMPLES

Percentages are by weight, unless otherwise stated.

THA is tetrahydroazepine.

The product mixtures were analyzed by gas chromatography. THAconcentrations below 20 ppm were determined by polarography.

Inventive Example

50 kg/h of HMD having a THA content of 71 ppm were fed at a uniform rateto a distillation column having 50 theoretical plates and 41 kg/h ofoverhead product and 9 kg/h of bottom product were removed from thedistillation apparatus at a reflux ratio of 1, a base-of-column pressureof 73 mbar and a pot temperature of 119.9° C.

The overhead product as well as HMD included 12 ppm of THA, the bottomproduct 334 ppm of THA.

Comparative Example

The inventive example was repeated except that the base-of-columnpressure was 255 mbar and the pot temperature 153.5° C. The overheadproduct as well as HMD included 44 ppm of THA, the bottom product 172ppm of THA.

1. A process for distillative removal of part or all of an azepinederivative (III) selected from the group consisting oftetrahydroazepine, 2-aminoazepan, N-(2-azepano)-1,6-diaminohexane andN-(2-azepano)-6-aminocapronitrile from a mixture (II) comprising anazepine derivative (III) and an amine (I), which comprises conductingthe distillation at a pot temperature of not more than 120° C., andwherein the distillation provides said azepine derivative (III)predominantly as bottom product (VI) and said bottom product (VI) issubjected to a hydrogenation.
 2. A process as claimed in claim 1,wherein said bottom product (VI) is used in a hydrogenation to preparean amine (I).
 3. The process of claim 1, wherein the amine (I) is6-aminocapronitrile or hexamethylenediamine.
 4. A process fordistillative removal of part or all of an azepine derivative (III)selected from the group consisting of tetrahydroazepine, 2-aminoazepan,N-(2-azepano)-1,6-diaminohexane and N-(2-azepano)-6-aminocapronitrilefrom a mixture (II) comprising an azepine derivative (III) and an amine(I), which comprises conducting the distillation at a pot temperature ofnot more than 120° C., and wherein the distillation is carried out intwo stages, the pressure in the first stage, measured in the pot, is atleast 1.5 times the pressure in the second stage, measured in the pot,not less than 20% by weight of the amount fed into the first stage perunit time is removed from the pot of the first stage and fed to thesecond stage; and the overhead product of the second stage is recycledinto the first stage.
 5. The process of claim 4, wherein the amine (I)is 6-aminocapronitrile or hexamethylenediamine.
 6. A process ofdistillative removal, of part or all of an azepine derivative (III)selected from the group consisting of tetrahydroazepine, 2-aminoazepan,N-(2-azepano)-1,6-diaminohexane and N-(2-azepano)-6-aminocapronitrilefrom a mixture (II) comprising an azepine derivative (III) and an amine(I), which comprises conducting the distillation at a pot temperature ofnot more than 120° C.,and wherein the distillation is carried out in thepresence of carbon dioxide.
 7. The process of claim 6, wherein the amine(I) is 6-aminocapronitrile or hexamethylenediamine.